1. Technical Field
The present invention relates to a circuit for maintaining the rotational speed of a motor at a constant, and more particularly to improvements in a current control type motor rotational speed regulator including a negative impedance, such that even if variations in the power supply voltage should occur, the rotational speed of the motor does not vary.
2. Background Art
In a current control type motor rotational speed regulator including a negative impedance as disclosed in U.S. Pat. No. 4,227,127, commonly owned, a voltage supply terminal of a power supply is connected to an input terminal of a control circuit through a resistor having a resistance proportional to an internal resistance of the motor, while the opposite terminals of the motor are connected respectively to the voltage supply terminal of the power supply and to an output terminal of the control circuit. Within the control circuit, the voltage applied to the input terminal has been subjected to a constant voltage drop, and then the dropped voltage is compared with the voltage at the output terminal. By this compared output, the current values are controlled so that the same current value may be applied to both the motor and the resistor to produce a constant counter electro-motive-force generated by the rotation of the motor.
Under variations of the load applied to the motor, the above-described motor rotational speed regulator can maintain a constant rotational speed regardless of the variation of the load, as a result of the fact that variation in the counter electro-motive-force of the motor is detected by the comparator, and the currents flowing through the motor and the resistor are controlled so as to keep the counter electro-motive-force constant. Thereby, the rotational speed of the motor is regulated so that the counter electro-motive-force may take a preset constant voltage value, as fully described in the specification of the above U.S. patent. However, when the voltage of the power supply varies, the current flowing through the resistor varies and consequently a constant rotational speed cannot be obtained. In other words, the above-mentioned motor speed regulator of the prior art has a disadvantage in that the rotational speed of the motor varies in accordance with variations in the voltage of the power supply.